In a radio transmitter, optimization of a power amplifier (PA) generally requires different load impedances to maximize efficiency and hence improve battery life. If a PA is to operate at different power levels, a lower load impedance is used for a high power (HP) mode, while a higher impedance is required to optimize PA efficiency for a low power (LP) mode. If a transmitter does not have means to adjust the impedance level, typical design considerations dictate that the PA be optimized only for a maximum supported power level. Instead, lower power level transmission is achieved by backing off PA power by way of gain control. Backing off PA power, however, reduces the PA efficiency, degrading battery life.
Many transmitters include an on-chip LC network to provide different impedance levels at the PA output. However, use of on-chip LC networks results in larger die area, increasing cost and size. Moreover, on-chip LC networks are generally quite lossy, which induce higher loss throughout a transmitter signal chain, causing reduced PA efficiency. As such, current transmitters suffer from inefficiencies.